1. Technical Field
The present invention relates to a piezoelectric vibration reed, a piezoelectric vibrator, an oscillator, an electronic instrument, and a radio timepiece.
2. Description of the Related Art
In mobile phones or portable information terminal equipment, a piezoelectric vibrator using crystal or the like is employed as a timing source or a reference signal source of a time-of-day source or a control signal. Various types of the piezoelectric vibrators of this type are provided, and as one of these piezoelectric vibrators, a piezoelectric vibrator having so-called a tuning-fork-type piezoelectric vibration reed is known.
The tuning-fork-type vibration reed includes a pair of vibrating arm portions disposed in parallel and a base portion configured to support proximal end portions of the pair of vibrating arm portions.
Then, an electrode film is formed on the surface of the piezoelectric vibration reed, and when a voltage is applied to the electrode film, the pair of vibrating arm portions can be brought into vibrations at a predetermined resonant frequency in the directions toward and away from each other.
In recent years, downsizing of the piezoelectric vibration reed is desired in association with downsizing of equipment having the piezoelectric vibrator mounted thereon.
However, when the width of the vibrating arm portions is reduced for example, the width of the electrode films (excitation electrodes) formed on the vibrating arm portions is also reduced. Consequently, the equivalent series resistance value thereof (the CI (Crystal Impedance) value) is increased, and the quality of an output signal may be deteriorated.
A resonant frequency F of the vibrations of the pair of vibrating arm portions is expressed by:F=k(W/L2)(k: coefficient, W: width of the vibrating arm portions, L: length of the vibrating arm portions). Therefore, when the length or the width of the vibrating arm portions is changed, the resonant frequency F may be shifted. Therefore, in order to achieve downsizing of the piezoelectric vibration reed, attention should be paid so as not to cause the resonant frequency to be shifted.
Therefore, formation of groove portions on upper and lower surfaces of the vibrating arm portions is known when reducing the width of the vibrating arm portions (see JP-A-2009-81520). When the grove portions are formed, the excitation electrodes in pair oppose each other between both sides of the groove portions. Therefore, efficient action of an electrical field in the opposing direction is achieved. Accordingly, enhancement of the electrical field efficiency is achieved even when the width of the vibrating arm portions is reduced, so that downsizing is achieved while improving the quality.
However, when the groove portions are formed on the vibrating arm portions, rigidity of the vibrating arm portions may be lowered. In particular, when the groove portions are formed in the vicinity of connecting portions between the proximal end portions of the vibrating arm portions and the base portion, a sufficient strength of the vibrating arm portions cannot be obtained and, in addition, a stress concentration on these portions may easily occur. Therefore, when an external impact or the like is applied on the piezoelectric vibration reed, the piezoelectric vibration reed is susceptible to breakage or the like starting from the portions in the vicinity of the connecting portions of the proximal end portions between the vibrating arm portions and the base portion.
The electric field efficiency is improved in proportion to the increase in width of the groove portions when the width of the vibrating arm portions is the same. In contrast, however, the larger the width of the groove portions, the more the rigidity of the vibrating arm portions in the direction of distortion or the direction of thickness thereof is decreased, so that the vibrating arm portions are susceptible to deformation. Consequently, the vibrations might be excited in a vibration mode different from a desired vibration mode, which may results in deterioration of the quality of the output signal.